Would this work? The spline decal looks like it would do the job.
I can think of some other ways that this can be done too:
- If your play area is a bunch of tiles, you could create a mesh for each edge of a tile, and then just show or hide those pieces based on the area a player controls and what each tile is connected to. Ie. If your tile is a hexagon, it will have 6 border pieces. If an edge is connected to the tile of another territory, show the piece, otherwise hide it.
- You could have a post-processing material that highlights the edges of something. This could be controlled by setting tags on the actors you want to highlight.